The present invention relates in general to electrooptic, alpha-numeric displays and, more particularly, to a reflective display which utilizes ferroelectric ceramic materials, such as polished plates of PLZT arranged along with polarizers, transparent electrodes, and an appropriate reflecting surface in order to achieve an optimum combined effect of high contrast, increased brightness, wide viewing angle, and reduced operating voltage requirements for the display. The result is a display which has an improved appearance and viewing angle and which operates at lower voltage requirements.
Ferroelectric ceramic materials such as PLZT, a well-known ceramic material which has inherently high optical transparency and electrically-controllable light modulation properties, have been found to be useful in optical shutter and display devices In these ferroelectric ceramic materials, light modulation may be produced by essentially two means: (1) light scattering, which has not proven to be a desirable display means because the achievable contrast ratios are too low for acceptable character recognition in high ambient light environments, and (2) optical birefringence, also known as the electrooptic effect, which is a material phenomenon which is capable of producing very high contrast ratios on the order of 5,000 to 1 and is very effective in producing easily viewable displays under all types of lighting conditions
In the prior art, the utilization of ferroelectric ceramic materials in various displays has occurred and has resulted in the generation of light modulation by both the light scattering effect approach and the optical birefringence method. In evaluating the technical embodiments of these approaches, it is helpful to distinguish between the essential elements of each approach. For instance, whether the material utilized is of single crystal structure or of ceramic materials. This is important because single crystal materials are distinguishable and different from the ceramic materials in that ceramic materials are composed of random aggregates of microscopic crystallites, on the order of 1 to 15 micrometers in average diameter, intimately bonded or sintered together to form a dense solid material. Likewise, the composition in grain size of the ferroelectric ceramic material utilized in displays found in the prior art will vary relative to the chemical ingredients and additives of the composition resulting in the properties of the ceramics, in some instances, approximating those of a single crystal of similar composition. While in many other instances, new and different properties of the ferroelectric ceramic materials will be created by virtue of the inherent grain structure of the crystallites and the inherent grain boundaries which result.
Any subtle differences in the selection of ceramic material or element placement in the display apparatus will result in the electrooptic behavior of the display device having large variations in the observed effect relative to known devices. For example, light scattering effects are minimal to non-existent in small grain-size materials (less than 2 microns) whether the material used is of the memory type or the non-memory type. Whereas, birefringence is observable and useful in both large grain size non-memory materials (greater than 2 microns), or small grain size non-memory materials; but birefringence is useful only in small grain size ceramic materials which are of the memory type. Additionally, optical transparency is enhanced when large grain size material is utilized and, therefore, it is most critical to a successful display apparatus, to utilize large grain size ceramic material in a non-memory display device, as for example, in a display device which requires maximum brightness and contrast.
It is well understood that the placement of the associated conductive electrodes on the ceraric material is critical to producing the desired display effect. The electrical field vector, which is determined by the placement of the electrodes, does define the internal polarization direction of the ceramic material, i.e., the atomic unit cell elongation, which affects the preferential light scattering direction or the unique optical birefringence retardation direction in the ceramic material utilized. The PLZT ceramics are optically birefringent, uniaxial materials, which are transparent in the wavelength region from 0.37 to 6.5 micrometers (neglecting reflection losses of approximately 32 percent for the combined two major surfaces of the PLZT plates utilized in the subject matter of the present display). The PLZT plates are defined as uniaxial because they possess one unique direction, i.e., the polarization direction, along which light travels at a different velocity relative to the other two orthogonal directions. It is important to recognize that PLZT ceramics possess optically uniaxial properties on a microscopic scale and also on a macroscopic scale when polarized with an electrical field. In uniaxial crystals, there is one unique symmetry axis, the optic axis, which is colinear with the ferroelectric polarization vector in the PLZT ceramics and which possesses different optical properties than the other two orthogonal axes. That is, light traveling in a direction along the optic axis, and vibrating in a direction perpendicular to the optic axis, encounters a different index of refraction than the light traveling in a direction at right angles to the optic axis and vibrating parallel to the optic axis. The difference in velocities, or indexes of refraction, is known as the birefringence or .DELTA.n, (where n=c/v where c=velocity of light in a vacuum, and v is the velocity of light along a given crystalline direction). Stated another way, the absolute difference between the two indices is defined as the birefringence, i.e., n.sub.E -n.sub.0 =.DELTA.n. The larger the .DELTA.n of a material, the greater is the inherent optical activity and ability to produce the desired optical retardation or phase delay. On a macroscopic scale, .DELTA.n is equal to zero before electrical poling and has some finite value after electrical poling, depending on the composition of the ceramic material utilized and the degree of polarization. The .DELTA.n value is a meaningful quantity in that it is related to the optical phase retardation in the ceramic material. For certain compositions within the PLZT materials, i.e., ferroelectric non-memory type ceramic material such as 9/65/35 (9% La, 65% PbZrO.sub.3 and 35% PbTiO.sub.3), .DELTA.n is electrically induced and is proportional to the square of the electrical field strength. This results in a quadratic ceramic material, since .DELTA.n=kE squared. The subject matter of the present patent application utilizes such ceramic materials.
These ferroelectric ceramic materials, by virtue of their natural cubic symmetry, do not possess permanent polarization and are not optically birefringent in their quiescent state. Such PLZT ceramic materials contribute no optical retardization to an incoming light beam. However, when an electrical field is applied to the PLZT ceramic materials, electrical polarization and birefringence is induced in the ceramic materials, and optical retardation is observed between cross-polarizers. Linearly polarized light, on entering the electrically energized ceramic material, is resolved into two perpendicular components, whose vibration directions are defined by the crystallographic axes of the crystallites acting as one optical entity. Because of the different refractive indices, n.sub.E and n.sub.O (i.e., the respective index along the propagation direction and the respective index perpendicular to the propagation direction), the propagation velocity of the two components will be different within the ceramic material and will result in a phase shift called retardation. The total retardation .GAMMA. is a function of both .DELTA.n and the optical path length t (i.e., the thickness of the ceramic PLZT plate), according to the relation .GAMMA.=.DELTA.n.times.t. When sufficient voltage is applied to the PLZT ceramic material, a half-wave retardation is achieved for one component relative to the other. The net result is one of rotating the vibration direction of the linearly polarized light by 90 degrees, thus allowing it to be transmitted by the second (or crossed) polarizer in the "ON" condition. Switching from the state of zero retardation to half-wave retardation will create a light shutter or an electrooptic display.